It is conventionally known that ultrasonic speakers using a non-linear effect of the medium (i.e., air) on an ultrasonic wave (signal) can reproduce a signal in an audio (i.e., human-audible) frequency band, which has far higher directivity in comparison with normal speakers. Representative examples of the ultrasonic speaker employ a resonant ultrasonic transducer or an electrostatic ultrasonic transducer.
FIG. 11A is a diagram showing an example of the structure of the resonant (or piezoceramic) ultrasonic transducer, while FIG. 11B is a diagram showing an example of the structure of the electrostatic ultrasonic transducer (refer to Ryousuke Masuda, “Hajimeteno Sensa Gijutsu”, Beginner's Books Series Vol. 2, Kogyo Chosakai Publishing Inc., pp. 131-133, Nov. 18, 1998).
The ultrasonic transducer shown in FIG. 11A is a bimorph ultrasonic transducer having two piezoceramic elements 161 and 162, a cone 163, a case 164, leads 165 and 166, and a screen 167. The piezoceramic elements 161 and 162 are adhered to each other, and the leads are respectively connected to the faces of the piezoceramic elements, on the opposite sides of the adhesion faces. The resonant transducer uses a resonance phenomenon of piezoelectric ceramics; thus, preferable ultrasonic transmitting and receiving characteristics are obtained in a relatively narrow frequency range in the vicinity of the resonance frequency.
The ultrasonic transducer shown in FIG. 11B is an electrostatic ultrasonic transducer having wide band frequency characteristics. As shown in FIG. 11B, the electrostatic ultrasonic transducer has a dielectric (material) 181 (i.e., an insulator) such as a PET polyethylene terephthalate) resin having a thickness of a few micrometers (approximately, 3 to 10 μm), as a vibrator. On the upper surface of the dielectric 181, an upper electrode 182, which is a foil made of metal, is integrally formed by vapor deposition or the like. In addition, a lower electrode 183 (a fixed electrode) made of brass is provided, which contacts the lower surface of the dielectric 181 which functions as a vibrating film or membrane. A lead 184 is connected to the lower electrode 183, and the lower electrode 183 is fastened to a base plate 185 made of Bakelite (a registered trademark of the Union Carbide Corporation) or the like. The dielectric 181, the upper electrode 182, and the base plate 185 are fixedly enclosed in a case 180, together with metal rings 186, 187, and 188, and a mesh 189.
On a surface of the lower electrode 183, which faces the dielectric 181, microgrooves having a (groove) width of approximately a few tens to a few hundreds of micrometers and having irregular forms are formed. The microgrooves function as gaps between the lower electrode 183 and the dielectric 181, which slightly change the distribution of electric capacitance between the upper electrode 182 and the lower electrode 183. Such microgrooves having irregular forms are formed by randomly scoring the surface of the lower electrode 183 with a file. Accordingly, the electrostatic ultrasonic transducer has an enormous number of capacitors having gaps whose areas and depths are not uniform, thereby rendering the ultrasonic transducer capable of producing sound in a wide frequency range in the frequency characteristics. The present invention uses an electrostatic ultrasonic transducer which will be explained in detail later.
As explained above, different from the resonant ultrasonic transducers, the electrostatic ultrasonic transducers are conventionally known as wide band transducers which can generate relatively high sound pressure over a wide frequency band.
However, when the above-explained electrostatic ultrasonic transducer is mounted into a projector so as to emit an ultrasonic wave signal onto a screen, the signal reflected by the screen may still include a strong ultrasonic wave due to strong directivity of the ultrasonic signal, and thus self-demodulation having directivity may occur after the reflection.
This phenomenon is not preferable for speakers used in projectors. More specifically, the reflected sound signal proceeds in the form of a beam and thus the spread of sound is reduced. This is a strong limitation when a number of people share images and sounds in a home theater or in an environment for the education/culture market, and a solution to this problem has been earnestly desired.